Artificial intervertebral disc having a bored semispherical bearing with a compression locking post and retaining caps

ABSTRACT

An artificial intervertebral disc having a pair of opposing baseplates, for seating against opposing vertebral bone surfaces, uses a semispherical, bored bearing that is secured to the baseplates with compression locking posts and one or more retaining caps. The compression locking posts extend through the bearing bore and baseplate apertures such that the bearing is between the baseplates&#39; inwardly facing surfaces. Retaining caps are attached to the compression locking posts, securing the baseplates to the bearing. Bearing surfaces on the inwardly facing side of each baseplate allow each baseplate to rotate relative to the bearing, however, rotation of each baseplate is limited by the interference of each baseplate and its respective retaining cap. Rotation of the baseplates about the longitudinal axis of the spine can be limited via a notch in the retaining caps and a groove in the baseplates, or vice versa.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/782,982, filed on Feb. 20, 2004 now U.S. Pat. No. 7,393,361, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to a device for implantation into anintervertebral space to simultaneously stabilize the adjacent vertebralbodies and permit proper anatomical motion at the segment. Specifically,the present invention relates to such a device having upper and lowerbaseplates that articulate about a central, bored semispherical bearing.The present invention maximizes the strength (i.e., compression andtension load capabilities) of such a device by allowing thesemispherical bearing to have a larger diameter without increasing theheight of the device.

The bones and connective tissue of an adult human spinal column consistof more than twenty discrete bones coupled sequentially to one anotherby a tri-joint complex, which consists of an anterior disc and twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than twenty bones are anatomically categorized as beingmembers of one of four classifications: cervical, thoracic, lumbar, orsacral. The cervical portion of the spine, which comprises the top ofthe spine up to the base of the skull, includes the first sevenvertebrae. The intermediate twelve bones are the thoracic vertebrae, andconnect to the lower spine comprising the five lumbar vertebrae. Thebase of the spine comprises the sacral bones (including the coccyx). Thecomponent bones of the cervical spine are generally smaller than thoseof the thoracic spine, which are in turn smaller than those of thelumbar region. The sacral region connects laterally to the pelvis.

The spinal column is highly complex in that it includes these more thantwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complications,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumors,and degenerative wear are a few of the causes that can result in spinalpathologies for which surgical intervention may be necessary. A varietyof systems have been disclosed in the art that achieve immobilizationand/or fusion of adjacent bones by implanting artificial assemblies inor on the spinal column. The region of the back that needs to beimmobilized, as well as the individual variations in anatomy, determinesthe appropriate surgical protocol and implantation assembly. Withrespect to the failure of the intervertebral disc, the interbody fusioncage has generated substantial interest because it can be implantedlaparoscopically into the anterior of the spine, thus reducing operatingroom time, patient recovery time, and scarification.

Referring now to FIGS. 2-3, in which a side perspective view of anintervertebral body cage and an anterior perspective view of a postimplantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 1 generally comprise tubular metal body 2 having an externalsurface threading 3. They are inserted transverse to the axis of thespine 4, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 3 the pair of cages 1 are inserted between thefifth lumbar vertebra (L5) and the top of the sacrum (S1)). Two cages 1are generally inserted side by side with the external surface threading3 tapping into the lower surface of the vertebral bone above (L5), andthe upper surface of the vertebral bone (S1) below. The cages 1 includeholes 5 through which the adjacent bones are to grow. Additionalmaterials, for example autogenous bone graft materials, may be insertedinto the hollow interior 6 of the cage 1 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 1.

These cages of the prior art have enjoyed medical success in promotingfusion and grossly approximating proper disc height. It is, however,important to note that the fusion of the adjacent bones is an incompletesolution to the underlying pathology as it does not cure the ailment,but rather simply masks the pathology under a stabilizing bridge ofbone. This bone fusion limits the overall flexibility of the spinalcolumn and artificially constrains the normal motion of the patient.This constraint can cause collateral injury to the patient's spine asadditional stresses of motion, normally borne by the now-fused joint,are transferred onto the nearby facet joints and intervertebral discs.It would therefore, be a considerable advance in the art to provide animplant assembly which does not promote fusion, but, rather, whichmimics the biomechanical action of the natural disc cartilage, therebypermitting continued normal motion and stress distribution.

It is, therefore, an object of the invention to provide anintervertebral spacer that stabilizes the spine without promoting a bonefusion across the intervertebral space.

It is further an object of the invention to provide an implant devicethat stabilizes the spine while still permitting normal motion.

It is further an object of the invention to provide a device forimplantation into the intervertebral space that does not promote theabnormal distribution of biomechanical stresses on the patient's spine.

It is further an object of the invention to provide an artificialintervertebral disc that supports compression loads.

It is further an object of the invention to provide an artificialintervertebral disc that supports tension loads.

It is further an object of the invention to provide an artificialintervertebral disc that prevents lateral translation of the baseplatesrelative to one another.

It is further an object of the invention to provide an artificialintervertebral disc that provides a centroid of motion centrally locatedwithin the intervertebral space.

It is further an object of the invention to provide artificialintervetebral disc that provides maximized strength without increasingthe height of the disc.

Other objects of the invention not explicitly stated will be set forthand will be more clearly understood in conjunction with the descriptionsof the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects are achieved by the invention, which is anartificial intervertebral disc or intervertebral spacer device,comprising a pair of support members (e.g., spaced apart baseplates),each with an outwardly facing surface. Because the artificial disc is tobe positioned between the facing endplates of adjacent vertebral bodies,the baseplates are arranged in a substantially parallel planar alignment(or slightly offset relative to one another in accordance with properlordotic angulation) with the outwardly facing surfaces directed awayfrom one another. The baseplates are to mate with the vertebral bodiesso as to not rotate relative thereto, but rather to permit the spinalsegments to bend or axially compress relative to one another in mannersthat mimic the natural motion of the spinal segment. This natural motionis permitted by the performance of a bearing disposed between thesecured baseplates, and the securing of the baseplates to the vertebralbone is preferably achieved through the use of a vertebral body contactelement attached to, or a surface feature of, the outwardly facingsurface of each baseplate.

Preferable body contact elements include, but are not limited to, aconvex mesh (of any shape or contour, but preferably domed) and one ormore spikes. These vertebral body contact elements are disclosed ingreater detail in application Ser. No. 10/256,160 (“the '160Application”) and application Ser. No. 10/642,258 (“the '258Application”), which are incorporated herein by reference.

To enhance the securing of the baseplates to the vertebral bones, eachbaseplate preferably further comprises a surface feature that permitsthe long-term ingrowth of vertebral bone into the baseplates. Apreferred surface feature is a porous area, which at least extends in aring around the lateral rim of each outwardly facing surface. The porousarea may be, for example, a sprayed deposition layer, an adhesiveapplied beaded metal layer, or another suitable porous coating known inthe art. The porous ring permits the long-term ingrowth of vertebralbone into the baseplates, thus permanently securing the prosthesiswithin the intervertebral space.

The semispherical bearing disposed between the baseplates permitsrotation and angulation of the two baseplates relative to one anotherand to the bearing, which establishes a centroid of motion (for thisrotation and angulation) centrally between the baseplates. Thesemispherical bearing is captured between the baseplates by first andsecond retaining caps which are connected together by engagement ofcompression locking posts. Further, the capturing prevents separationand/or disassembly of the device under tension loading, and preventslateral translation of the baseplates, during the rotation andangulation.

More specifically, the two baseplates of the present invention eachinclude an aperture and each is secured to a bored central bearing inthe following manner. The first and second baseplates are disposed suchthat their outwardly facing surfaces face away from one another, andtheir inwardly facing surfaces are directed toward one another. Thesecond baseplate aperture is then passed over the compression lockingpost of second retaining cap and integral second retaining cap such thatthe compression locking post passes through the outwardly facing surfacefirst and the inwardly facing surface second. A circumferentialprotrusion in the second baseplate aperture wall (i.e., the axiallyinwardly directed surface of the second baseplate) will rest upon theinwardly facing surface of the second retaining cap. Next, the bore ofthe central bearing is passed over the compression locking post and intothe second baseplate aperture until a portion of the bearing having asmaller diameter contacts the inwardly facing surface of the secondretaining cap and a portion of the bearing having a larger diametercontacts the inwardly facing surface of the circumferential protrusionin the wall of the second baseplate aperture. Then, the first baseplateaperture is passed over the compression locking post until thecircumferential protrusion in the first baseplate aperture wall (i.e.,the axially inwardly directed surface of the first baseplate) rests uponthe bearing. Finally, compression locking post of the first retainingcap is pressed into the bearing bore and over the compression lockingpost of the second retaining cap under a force sufficient to compressionlock the two compression locking posts, its integral retaining caps, andthe bearing. At this point, the two retaining caps, compression lockingposts, and bearing become one stationary unit (i.e., the retaining caps,compression locking posts, and bearing do not rotate or otherwise moverelative to each other). The baseplates are free to rotate andarticulate about the bearing and its firmly affixed retaining caps andpost).

After assembly, as described above, the inwardly facing surfaces of thebaseplate aperture walls (i.e., the surfaces extending from thecircumferential protrusion in each aperture wall to the inward edge ofeach aperture wall) provide bearing surfaces, within which the bearingis captured, thereby facilitating limited angulation of the baseplatesrelative to the bearing. These bearing surfaces are preferably contouredto closely accommodate the spherical contour defined by the bearing,such that the bearing may easily contact and slide against the bearingsurfaces. In this manner, the baseplate bearing surfaces, and thereforethe baseplates, may angulate with limitation about the bearing.

As noted above, angulation of the baseplates relative to the bearing islimited. The outwardly facing surfaces of the baseplate aperture walls(i.e., the surfaces extending from the circumferential protrusion ineach aperture wall to the outward edge of each aperture wall) aretapered to a larger diameter toward the baseplate's outwardly facingsurfaces. Additionally, and preferably, the conformation of the tapermatches the contour defined by the inwardly facing surface of therespective retaining cap. Because the retaining caps and posts arestationary with respect to the bearing, such tapering and conformationof the baseplate aperture wall permits the baseplates to angulate (aboutthe centroid of motion at the center of the bearing) with respect to thebearing until the point at which the baseplate interferes with, orcontacts, the respective retaining cap. Therefore, the taper, diameter,and conformation of this articulation (i.e., the space between theretaining cap and its respective baseplate) limit the angular movementof the respective baseplate relative to the bearing. Preferably, thetaper, diameter, and conformation of the taper accommodate rotation ofthe respective baseplate relative to the bearing at least until theinwardly facing surfaces of the baseplates meet.

Furthermore, in the preferred embodiment of the present invention, theaxial rotation of each baseplate is limited, preferably to between 7 and10 degrees. This limitation may be created using a variety of methods.For example, this can be realized by a notch and groove, wherein notchesare formed in each retaining cap and grooves are formed in eachbaseplate. Alternatively, the grooves may be formed in the retainingcaps and the notches may be formed in the baseplates.

Accordingly, the baseplates rotate with limitation relative to thebearing. Because the bearing is secured to the baseplates with thecompression locking posts and retaining caps as discussed above, theartificial intervertebral disc of the present invention can withstandtension loading of the baseplates, and the assembly does not come apartunder normally experienced tension loads. Thus, in combination with thesecuring of the baseplates to the adjacent vertebral bones, the discassembly has an integrity similar to the tension-bearing integrity of ahealthy natural intervertebral disc. Also because the bearing islaterally captured between the bearing surfaces, lateral translation ofthe baseplates relative to one another is prevented during rotation andangulation, similar to the performance of a healthy naturalintervertebral disc. The baseplates are designed to rotate relative tothe bearing, therefore, the disc assembly provides a centroid of motionwithin the bearing. Accordingly, the centroid of motion of the discassembly remains centrally located between the vertebral bodies, similarto the centroid of motion in a healthy natural intervertebral disc.

In addition to the features and functions described above for thebaseplate apertures, the present invention can take advantage of theconcavities of the adjacent vertebral bodies, and allow the size of thebearing, and accordingly its ability to withstand compression andtension loads, to be maximized. Specifically, the present invention isdesigned to allow each retaining cap to protrude beyond the outwardlyfacing surface of the respective baseplate into the concavity of thevertebral body adjacent to the outwardly facing surface, facilitatingrotation of the baseplate on the bearing 28. Moreover, enlargement ofthe bearing creates a more robust bearing assembly that is able towithstand greater compression and tension forces than the same bearingassembly having a smaller size.

It should be understood that each of the features of the preferred andalternate embodiments described herein, including, but not limited to,formations and functions of baseplates, manners of contacting thebearing ball with bearing surfaces, manners of limiting rotation of thebaseplates relative to one another, and manners of allowing the bearingmechanism to extend into the concavities of adjacent vertebral bodies,can be included in other embodiments, individually or with one or moreof the other features, in other permutations of the features, includingpermutations that are not specifically described herein, withoutdeparting from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-c show top (FIG. 1 a), side (FIG. 1 b), and bottom (FIG. 1 c)views of an assembled preferred embodiment of the present invention.

FIGS. 1 d-1 e show exploded (FIG. 1 d) and assembled (FIG. 1 e) views ofthe preferred embodiment of the present invention.

FIGS. 1 f-1 h show side cutaway perspective exploded (FIG. 1 f) and sidecutaway assembled perspective and straight (FIGS. 1 g and 1 h) views ofthe preferred embodiment of the present invention.

FIG. 2 shows a side perspective view of a prior art interbody fusiondevice.

FIG. 3 shows a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devices(as shown in FIG. 2) have been planted.

DETAILED DESCRIPTION

While the invention will be described more fully hereinafter withreference to the accompanying drawings, in which particular embodimentsand methods of implantation are shown, it is to be understood at theoutset that persons skilled in the art may modify the invention hereindescribed while achieving the functions and results of the invention.Accordingly, the descriptions that follow are intended to beillustrative and exemplary of specific structures, aspects, and featureswithin the broad scope of the invention and not as limiting of suchbroad scope.

Referring first to FIGS. 1 g and 1 h, the artificial intervertebral discassembly of the present invention generally comprises first and secondendplates 10, 30 rotatably retained on a bearing 28 of a bearingmechanism which comprises bearing 28 and first and second retaining caps12, 34. When assembled as shown in FIGS. 1 g and 1 h, retaining caps 12,34 are connected to each other by engagement of respective compressionlocking posts 64, 14, whereby retaining caps 12, 34 and bearing 28 forman integrated piece and cannot move relative to one another, which willbe explained in greater detail.

Referring also to FIGS. 1 a-c, the assembled artificial intervertebraldisc of the present invention is shown in top (FIG. 1 a), side (FIG. 1b), and bottom (FIG. 1 c) views. Generally, first baseplate 10 andsecond baseplate 30 rotate relative to bearing 28, which is capturedbetween baseplates 10, 30 via first retaining cap 12 and secondretaining cap 34. This capturing is accomplished by a compressionlocking post 14 of second retaining cap 34 and a compression lockingpost 64 of first retaining cap 12 compression locking to one another(via a bore 38 in compression locking post 64) and into axial bores 56,60 in bearing 28 (See also FIGS. 1 d-h.).

More specifically FIG. 1 a depicts a top view of first baseplate 10,first retaining cap 12, compression locking post 14, first baseplatebeveled edges 16, and first baseplate aperture 18 of the preferredembodiment of the present invention.

First baseplate 10, as well as second baseplate 30 (FIGS. 1 b-c), aresolid baseplates preferably comprised of a metal or metal alloy, such asa metal alloy including cobalt-chromium. However, baseplates 10, 30 mayalso be comprised of other types of metal or non-metal materials withoutdeparting from the scope of the present invention.

As shown in FIG. 1 d, compression locking post 64 of first retaining cap12 has a compression locking post aperture 38 (FIG. 1 d) slightlysmaller in diameter than the outer diameter of compression locking post14. This dimensional difference allows compression locking post 14 to beattached to compression locking post 64 of first retaining cap 12 via acompression lock (i.e., forcing compression locking post 14 intocompression locking post 64 of first retaining cap aperture 38 viaapplication of pressure, such that the two cannot be separated absent aseparation force greater than those experienced under spinal loads thatcan be survived by the patient). Alternatively or in addition to acompression locking attachment method, other methods of attachingcompression locking post 14 to compression locking post 64 of firstretaining cap 12 may be incorporated, including, but not limited to,laser welding (i.e., the laser weld may be applied from the outwardlyfacing surface of compression locking post 64 of first retaining cap 12at the point where it contacts compression locking post 14), threading,etc. Thus, first and second retaining caps 12, 34 and bearing 28sandwiched therebetween form an integrated piece and cannot moverelative to one another. The integration of caps 12, 34 and bearing 28can also be realized or enhanced by compression locking (or threadingengagement, etc) between posts 14, 64 and bearing bores 60, 56.

Referring to FIGS. 1 d and 1 f, first baseplate 10 includes firstbaseplate outwardly facing surface 22 and first baseplate aperture 18.The aperture wall (i.e., the axially inwardly directed surface) of firstbaseplate 10 contains a first baseplate circumferential protrusion 40(FIG. 1 f) that retains first baseplate 10 between first retaining cap12 and bearing 28 while allowing first baseplate 10 to rotate relativeto bearing 28, as described in greater detail below with respect to FIG.1 f.

Referring next to FIG. 1 b, shown is a side view of baseplates 10, 30and bearing 28. Also depicted in FIG. 1 b, first baseplate inwardlyfacing surface 20 is flat, and first baseplate outwardly facing surface22 is shaped as a convex dome. Similarly, second baseplate inwardlyfacing surface 24 is flat, and second baseplate outwardly facing surface26 is shaped as a convex dome. Although the outwardly facing surfaces22, 26 of baseplates 10,30 of the preferred embodiment of the presentinvention are preferably shaped as convex domes so as to match the shapeof the endplates of adjacent vertebral bodies to which the baseplates10, 30 are to be attached, it should be noted that the outwardly facingsurfaces of the baseplates are not limited to this particular shape.Also, as depicted, because clearance between retaining caps 12, 34 andbaseplates 10, 30 allows rotation of the baseplate on the bearing 28,retaining caps 12, 34 may protrude slightly from baseplate outwardlyfacing surfaces 22, 26 during such rotation. After insertion of thedevice between vertebral bodies, retaining caps 12,34 protrude slightlyinto the concavities of vertebral bodies located adjacent to baseplates10, 30, respectively.

Since the artificial disc of the present invention is to be positionedbetween the facing surfaces of adjacent vertebral bodies, baseplates 10,30 of the present invention are disposed such that baseplate outwardlyfacing surfaces 22, 26 face away from one another as best illustrated inthe assembly view in FIG. 1 d. Baseplate outwardly facing surfaces 22,26 include first baseplate beveled edges 16 (FIG. 1 a) and secondbaseplate beveled edges 36 (FIG. 1 c), respectively, and are designed toconform to the overall shape of the respective endplates of thevertebral bodies with which they will mate.

Preferably, baseplate outwardly facing perimeter regions 17,33 (FIG. 1a, c) of baseplate outwardly facing surfaces 22, 26 are osteoconductivedue to, for example, a sprayed deposition layer, an adhesive appliedbeaded metal layer, or a similar suitable porous coating that is appliedto these surfaces using methods known in the art. These baseplateoutwardly facing surfaces 22, 26 permit the long-term ingrowth ofvertebral bone into baseplates 10, 30, thus permanently securing theartificial intervertebral disc within the intervertebral space. Thematerial applied to create the osteoconductive baseplate outwardlyfacing perimeter regions 17,33 of baseplate outwardly facing surfaces22, 26 may extend closer to baseplate apertures 18, 32. However, it ismost important that this osteoconductive material is applied to theportions of baseplate outwardly facing surfaces 22, 26 that seatdirectly against the adjacent vertebral body.

An alternate embodiment of the present invention may include one or morevertebral body contact elements including, but not limited to, a convexmesh, a convex dome, and one or more spikes as disclosed in the '160 and'258 Applications. These elements could be attached to baseplateoutwardly facing surfaces 22, 26, also as described in the '160 and '258Applications.

It should also be noted that depending upon the magnitude of expansionor contraction of the baseplates relative to each other, if any, firstretaining cap 12 and second retaining cap 34, might protrude outwardfrom the baseplate outwardly facing surfaces 22, 26, respectively. Itshould be further noted that the convex mesh, also disclosed in the '160Application, is suitable for use with the present invention, andpreferably should be attached to baseplate outwardly facing surfaces 22,26, outside of the area of motion of retaining caps 12, 34. Suchattachment may be performed via a variety of methods including, but notlimited to laser welding, or more preferably, plasma burying (i.e., theperimeter region of the convex mesh is buried under a plasma coating,which coating secures to the outwardly facing surface of the baseplateto which it is applied, and thus secures the convex mesh to theoutwardly facing surface). Preferably, the convex mesh has a concavitysuch that contact with retaining caps 12,34 is avoided.

Baseplates 10, 30 are designed to mate with the vertebral bodies suchthat they do not rotate relative thereto, but rather permit the spinalsegments to bend relative to one another in manners that mimic thenatural motion of the spinal segment. This motion is permitted by theperformance of bearing 28 disposed between baseplates 10, 30, which aresecured thereto via compression locking posts 14, 64 (FIG. 1 d), firstretaining cap 12 (FIG. 1 a), and second retaining cap 34 (FIG. 1 c).

Baseplates 10, 30 are joined with bearing 28, first retaining cap 12,and second retaining cap 34. In a preferred embodiment of the presentinvention, bearing 28 has a semispherical shape, however, other shapesmay be incorporated without departing from the scope of the presentinvention. Each of baseplates 10, 30 includes bearing surfaces 70, 74(FIG. 1 f), respectively, within which bearing 28 is capturable to allowlimited rotation of baseplates 10, 30 relative to bearing 28. Eachbearing surface is semispherically contoured to closely accommodate andengage with the spherical contour defined by bearing 28, such thatbaseplates 10, 30 may rotate transverse to the axis of the spine and mayrotate relative to bearing 28 about a centroid of motion located at thecenter of bearing 28. As illustrated in FIG. 1 f, bearing 28 includesfirst bearing bore 56, which accepts compression locking post 64protruding from first retaining cap 12 (FIG. 1 d), as well as secondbearing bore 60, which accepts compression locking post 14 inserted intocompression locking post aperture 38 of compression locking post 64.Preferably, compression locking post 64 locks into first bearing bore 56via a compression lock. In the preferred embodiment of the presentinvention, the bearing bore comprises a first bearing bore 56 and asecond bearing bore 60, each section having a different diameter,however, an alternate embodiment of the present invention may include asingle bearing bore having a single, consistent diameter.

In the preferred embodiment of the present invention, the diameter ofbearing 28 is slightly larger than the diameters of baseplate apertures18, 32 (FIGS. 1 a, 1 c), such that during axial compression ofbaseplates 10, 30 and no, or minimal, angular rotation of either ofbaseplates 10, 30, baseplate bearing surfaces 70, 74 directly contactaxially outwardly directed bearing surface 48, and baseplate inwardlyfacing surfaces 20, 24 do not make contact. Therefore, in an axiallycompressed state, baseplates 10, 30, and the vertebral bodies adjacentthereto, retain the ability to rotate relative to bearing 28. Thisallowed rotation mimics that found in the corresponding sections of anatural spine.

Referring next to FIG. 1 c, shown are bottom views of second baseplate30, second baseplate aperture 32, second baseplate outwardly facingperimeter region 33, second retaining cap 34, and second baseplatebeveled edges 36. In the preferred embodiment of the present invention,second retaining cap 34 and compression locking post 14 (FIG. 1 a) aremanufactured as a single component. However, in an alternate embodimentof the present invention, second retaining cap 34 and compressionlocking post 14 are manufactured as separate components and are affixedto each other during assembly of the present invention. The methods ofattachment include, but are not limited to, compression locking andthreading.

Turning next to FIGS. 1 d and 1 e, shown are an exploded view (FIG. 1 d)and an assembly view (FIG. 1 e) of the preferred embodiment of thepresent invention. Assembly of the artificial intervertebral disc is asfollows. Baseplates 10, 30 are disposed such that their baseplateoutwardly facing surfaces 22, 26, respectively, face away from oneanother and their baseplate inwardly facing surfaces 20, 24,respectively, are directed toward one another. Second baseplate aperture32 is then passed over compression locking post 14 and integral secondretaining cap 34 such that compression locking post 14 passes throughsecond baseplate outwardly facing surface 26 first and through secondbaseplate inwardly facing surface 24 second, and until the secondretaining cap inwardly facing tapered surface 80 is in contact withtapered second baseplate outwardly facing aperture wall surface 76 (FIG.1 f) (i.e., the surface extending from second baseplate circumferentialprotrusion 46 (FIG. 1 f) to the outward edge of the aperture wall).Next, second bearing bore 60 (FIG. 1 f), and, consequently, firstbearing bore 56 (FIG. 1 f), are passed over compression locking post 14until outwardly facing bearing surface 50 contacts inwardly facingsecond retaining cap surface 52 of second retaining cap 34 and axiallyoutwardly directed bearing surface 48 contacts second baseplate bearingsurface 74 (FIG. f). Then, first baseplate aperture 18 is passed overcompression locking post 14 until first baseplate circumferentialprotrusion 40 (FIG. 1 f) and first baseplate bearing surface 70 contactaxially outwardly directed bearing surface 48. Finally, compressionlocking post 64 of first retaining cap 12 is passed over compressionlocking post 14 into first bearing bore 56 (FIG. 1 f) under a forcesufficient to radially compress compression locking post 14 and radiallyexpand first bearing bore 56 (FIG. 1 f). Force is applied until inwardlyfacing first retaining cap surface 54 is in contact with second bearingbore inwardly facing surface 58 (FIG. 1 f) of second bearing bore 60(FIG. 1 f). After the force is removed, the radial pressure exerted bycompression locking post 14 on first retaining cap axially inwardlydirected surface (inner surface) 62 (FIG. 1 f) of compression lockingpost 64 of first retaining cap 12, as well as the radial pressureexerted by compression locking post 64 on first bearing bore 56 (FIG. 1f), acts to lock first retaining cap 12, bearing 28, and secondretaining cap 34, such that these components do not separate and do notrotate or, otherwise move, relative to each other.

Referring next to FIGS. 1 f-1 h, shown are a side cutaway exploded view(FIG. 1 f) and a side cutaway assembly view (FIGS. 1 g and 1 h) of thepreferred embodiment of the present invention. These side cutaway viewsdepict the internal dimensions of each of the components of the presentinvention as well as the assembled configuration of each componentrelative to the other components.

As depicted in FIG. 1 f, first retaining cap 12 comprises outward firstretaining cap section 66, first retaining cap inwardly facing taperedsurface 68, and compression locking post 64, which has a smallerdiameter than outward first retaining cap section 66 [see above].Similarly, first baseplate 10 has first baseplate circumferentialprotrusion 40, having the smallest diameter of any portion of firstbaseplate 10, and tapered first baseplate outwardly facing aperture wallsurface 72, which is tapered to mate with first retaining cap inwardlyfacing tapered surface 68. Furthermore, first baseplate 10 has a firstbaseplate bearing surface 70 having a concavity equivalent, or nearequivalent, to the contour defined by bearing 28.

Similarly, also as depicted in FIG. 1 f, compression locking post 14with integral second retaining cap 34 comprises outward second retainingcap section 78 and second retaining cap inwardly facing tapered surface80. Second baseplate 30 has a second baseplate circumferentialprotrusion 46 having the smallest diameter of any portion of secondbaseplate 30, and tapered second baseplate outwardly facing aperturewall surface 76 tapered to mate with second retaining cap inwardlyfacing tapered surface 80. Furthermore, second baseplate 30 has a secondbaseplate bearing surface 74 having a concavity equivalent, or nearequivalent, to the contour defined by bearing 28.

Accordingly, due to these configurations, the baseplates 10, 30 are ableto rotate relative to bearing 28. The semispherical contour of firstbaseplate bearing surface 70 closely matches the spherical contourdefined by bearing 28, such that first baseplate 10 can rotate about thecentroid of motion located at the center of bearing 28. Further, taperedfirst baseplate outwardly facing aperture wall surface 72 is tapered toa larger diameter toward the first baseplate outwardly facing surface22. Additionally, and preferably, the conformation of the taper matchesthe contour defined by first retaining cap inwardly facing taperedsurface 68. Since first retaining cap 12 and compression locking post 64are stationary with respect to bearing 28, such tapering andconformation of tapered first baseplate outwardly facing aperture wallsurface 72 permits first baseplate 10 to rotate (about the centroid ofmotion at the center of bearing 28) with respect to bearing 28 until thepoint at which first baseplate 10 interferes with, or contacts, thefirst retaining cap inwardly facing tapered surface 68. Therefore, thetaper, diameter, and conformation of these interacting elements (i.e.,the formation of the space between first retaining cap 12 and firstbaseplate 10) can be established to limit the rotational ability of thefirst baseplate 10 relative to bearing 28. Preferably, the taper,diameter, and conformation of these interacting elements accommodaterotation of first baseplate 10 relative to bearing 28 at least untilbaseplate inwardly facing surfaces 20, 24 of baseplates 10, 30 meet. Inother words, the ability of first baseplate 10 to rotate relative tobearing 28 is limited by the distance between first retaining capinwardly facing tapered surface 68 and first baseplate 10, as well asthe distance between baseplates 10, 30.

Similarly, the semispherical contour of second baseplate bearing surface74 closely matches the spherical contour defined by bearing 28, suchthat bearing 28 can rotate about the about a centroid of motion locatedat the center of bearing 28. Further, tapered second baseplate outwardlyfacing aperture wall surface 76 is tapered to a larger diameter towardthe second baseplate outwardly facing surface 26. Additionally, andpreferably, the conformation of the taper matches the contour defined bysecond retaining cap inwardly facing tapered surface 80. Since secondretaining cap 34 and compression locking post 14 are stationary withrespect to bearing 28, such tapering and conformation of tapered secondbaseplate outwardly facing aperture wall surface 76 permits secondbaseplate 30 to rotate (about the centroid of motion at the center ofbearing 28) with respect to bearing 28 until the point at which secondbaseplate 30 interferes with, or contacts, the second retaining capinwardly facing tapered surface 80. Therefore, the taper, diameter, andconformation of these interacting elements (i.e., the formation of thespace between second retaining cap 34 and second baseplate 30) can beestablished to limit the rotational ability of the second baseplate 30relative to bearing 28. Preferably, the taper, diameter, andconformation of these interacting elements accommodate rotation ofsecond baseplate 30 relative to bearing 28 at least until baseplateinwardly facing surfaces 20, 24 of baseplates 10, 30 meet. In otherwords, the ability of second baseplate 30 to rotate relative to bearing28 is limited by the distance between second retaining cap inwardlyfacing tapered surface 80 and second baseplate 30, as well as thedistance between baseplates 10, 30.

In the preferred embodiment of the present invention, the axial rotationof baseplates 10, 30 (about the longitudinal axis of the spine) isunlimited. In other embodiments, the axial rotation is limited,preferably to from 7 to 10 degrees. This limitation may be created usinga variety of methods including, for example, a notch formed in retainingcaps 12, 34 and a groove formed in baseplates 10, 30. Alternatively, thegroove may be formed in retaining caps 12,34 and the notch may be formedin baseplates 10, 30.

As best shown in FIGS. g and 1 h, clearance exists between thebaseplates 10, 30 and the bearing 28, as well as between the baseplates10, 30 and the retaining caps 12, 34, whereby the baseplates 10, 30 arenot only capable of rotating and angulating about the centroid of motionat the center of the bearing 28, but also capable of floating along theaxial direction relative to each other, thus realizing a universalmotion of the baseplates. When the baseplates float toward each other,the retaining caps 12 and 34 may protrude slightly beyond the outwardlyfacing surfaces of the baseplates 10, 30, and are accepted by the spacesformed by the concave contour of the endplates of the vertebral bodies.

The diameter of bearing 28, and its corresponding ability to withstandcompression and tension stress loads, can be increased without a need toincrease the height of the bearing 28 (which is limited by the spacingbetween adjacent vertebral bodies). Increasing the diameter of bearing28 also increases the bearing surface and reduces point loading.Consequently, a more robust artificial intervertebral disc is achievedthat is capable of withstanding the naturally occurring compression andtension forces exerted by adjacent vertebral bodies.

Whereas specific embodiments of an artificial intervertebral disc havebeen described and illustrated herein, it will be apparent to those ofskill in the art that variations and modifications to that disclosedherein are possible without deviating from the broad spirit, scope, andprinciples of the present invention. Therefore, the present inventionshall not be limited to the specific embodiments disclosed herein.

The invention claimed is:
 1. A method of assembling an artificialintervertebral disc comprising: inserting a first locking post of afirst retaining cap through a first baseplate aperture in a firstbaseplate and at least partially through a first bearing bore in abearing; and inserting a second locking post of a second retaining capthrough a second baseplate aperture in a second baseplate and at leastpartially through a second bearing bore in the bearing, the first andsecond bearing bores each having a different diameter, wherein the firstand second locking posts engage with one another and the first andsecond baseplates are rotatably retained on the bearing, such that thefirst and second baseplates and the bearing are configured to rotaterelative to one another after implantation of the artificialintervertebral disc into a patient, and wherein each of the first andsecond retaining caps has a flange.
 2. The method of claim 1, furthercomprising a step of mating the first locking post with the secondlocking post.
 3. The method of claim 2, wherein the mating step includesinserting the second locking post into a first retaining cap aperture ofthe first retaining cap.
 4. The method of claim 3, wherein the firstretaining cap aperture extends at least partially through the firstlocking post.
 5. The method of claim 3, wherein the mating step includescompression locking the second locking post within the first retainingcap aperture.
 6. The method of claim 3, wherein the mating step includeslaser welding the second locking post within the first retaining capaperture.
 7. The method of claim 1, further comprising steps ofcapturing the bearing within a first bearing surface formed on the firstbaseplate and capturing the bearing within a second bearing surfaceformed on the second baseplate.
 8. The method of claim 1, furthercomprising steps of arranging a first inwardly facing surface of thefirst baseplate and a second inwardly facing surface of the secondbaseplate so that they face one another and arranging a first outwardlyfacing surface of the first baseplate and a second outwardly facingsurface of the second baseplate so that they face away from one another.9. The method of claim 8, wherein the inserting steps include placing afirst retaining cap section of the first retaining cap adjacent thefirst outwardly facing surface and placing a second retaining capsection of the second retaining cap adjacent the second outwardly facingsurface.
 10. The method of claim 1, further comprising steps ofcontacting the bearing with a first circumferential protrusion of thefirst baseplate and a second circumferential protrusion of the secondbaseplate.
 11. The method of claim 1, wherein the inserting stepsinclude inserting the first locking post completely through the firstbearing bore formed in the bearing and inserting the second locking postcompletely through the second bearing bore formed in the bearing. 12.The method of claim 1, further comprising a step of rotating the firstbaseplate with respect to the bearing.
 13. The method of claim 12,further comprising a step of rotating the second baseplate with respectto the bearing.
 14. A method of assembling an artificial intervertebraldisc comprising: inserting a first locking post of a first retaining capthrough a first baseplate aperture in a first baseplate and at leastpartially through a bearing bore in a bearing; and inserting a secondlocking post of a second retaining cap through a second baseplateaperture in a second baseplate, at least partially through the bearingbore, and into a first retaining cap aperture, the first retaining capaperture extending completely through the first retaining cap, whereinthe first and second baseplates are rotatably retained on the bearing,such that the first and second baseplates and the bearing are configuredto rotate relative to one another after implantation of the artificialintervertebral disc into a patient, and wherein each of the first andsecond retaining caps has a flange.
 15. The method of claim 14, furthercomprising a step of compression locking the second locking post withinthe first retaining cap aperture.
 16. The method of claim 14, furthercomprising a step of laser welding the second locking post within thefirst retaining cap aperture.
 17. The method of claim 14, furthercomprising steps of capturing the bearing within a first bearing surfaceformed on the first baseplate and capturing the bearing within a secondbearing surface formed on the second baseplate.
 18. The method of claim14, further comprising steps of arranging a first inwardly facingsurface of the first baseplate and a second inwardly facing surface ofthe second baseplate so that they face one another and arranging a firstoutwardly facing surface of the first baseplate and a second outwardlyfacing surface of the second baseplate so that they face away from oneanother.
 19. The method of claim 18, wherein the inserting steps includeplacing a first retaining cap section of the first retaining capadjacent the first outwardly facing surface and placing a secondretaining cap section of the second retaining cap adjacent the secondoutwardly facing surface.
 20. The method of claim 14, further comprisingsteps of contacting the bearing with a first circumferential protrusionof the first baseplate and a second circumferential protrusion of thesecond baseplate.
 21. The method of claim 14, wherein the insertingsteps include inserting the first locking post into a first bearing boreformed in the bearing and inserting the second locking post into asecond bearing bore formed in the bearing.
 22. The method of claim 14,further comprising a step of rotating the first baseplate with respectto the bearing.
 23. The method of claim 22, further comprising a step ofrotating the second baseplate with respect to the bearing.
 24. A methodof assembling an artificial intervertebral disc comprising: inserting afirst locking post of a first retaining cap through a first baseplateaperture in a first baseplate and at least partially through a firstbearing bore in a bearing; inserting a second locking post of a secondretaining cap through a second baseplate aperture in a second baseplate,at least partially through a second bearing bore in the bearing, andinto a first retaining cap aperture of the first retaining cap; andcompression locking the second locking post within the first retainingcap aperture, wherein the first and second baseplates are rotatablyretained on the bearing, such that the first and second baseplates andthe bearing are configured to rotate relative to one another afterimplantation of the artificial intervertebral disc into a patient, andwherein each of the first and second retaining caps has a flange, eachof the flanges having a diameter that is greater than a diameter of thefirst and second bearing bores.
 25. The method of claim 24, furthercomprising steps of arranging a first inwardly facing surface of thefirst baseplate and a second inwardly facing surface of the secondbaseplate so that they face one another and arranging a first outwardlyfacing surface of the first baseplate and a second outwardly facingsurface of the second baseplate so that they face away from one another,wherein the inserting steps include placing a first retaining capsection of the first retaining cap adjacent the first outwardly facingsurface and placing a second retaining cap section of the secondretaining cap adjacent the second outwardly facing surface.